Can fingers point to sex habits?

Researchers measured the length of fossilized fingers from Ardipithecus and other ancestors on humanity's family tree, then compared them with modern-day species in hopes of figuring out how aggressive and promiscuous long-gone species might have been.

The study, published in the Proceedings of the Royal Society B, draws upon a famous and controversial indicator of social behavior: the comparative length of the index finger and the ring finger, also known as the 2D:4D ratio.

Emma Nelson, an archaeologist from the University of Liverpool, extended the finger-length ratio study to a wide range of species. She and her colleagues measured bones from modern-day species such as gorillas, chimps, orangutans and the white-handed gibbon. They also looked at fossil bones or previously recorded measurements from extinct hominids ranging from Neanderthals (which co-existed with humans until about 30,000 years ago) to Australopithecus afarensis (Lucy's species, going back 3.1 million years) and Ardipithecus ramidus (the oldest human-linked fossil species, going back 4.4 million years).

Nelson argues that comparing the finger-length ratios of extinct and present-day species is a valid technique for making an indirect assessment of our long-gone ancestors' social behavior.

"It is believed that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system," she explained in a news release. "We have recently shown that promiscuous primate species have low index-to-ring finger ratios, while monogamous species have high ratios. We used this information to estimate the social behavior of extinct apes and hominins."

Nelson previewed her findings a year ago at the Society of Vertebrate Paleontology's annual meeting, where she talked about naughty Neanderthals and monogamous australopiths. The newly published paper draws upon additional samples, including the finger lengths for Ardipithecus, or "Ardi." So here are the numbers:

Modern humans averaged a 0.957 index-to-ring finger ratio, and were considered to be on the line between a "pair-bonded," or monogamous, species and a middle-of-the-road species.

Chimps, gorillas and orangutans had index-to-ring ratios in the 0.90 to 0.92 range, and were classified as "non-pair-bonded," or promiscuous.

An early modern human from Israel's Qafzeh Cave, thought to be about 95,000 years old, had an index-to-ring ratio of 0.935. Based on that statistic, the researchers surmised this individual would be more promiscuous than modern humans.

The finger bones from five Neanderthals yielded a 0.928 ratio, associated with even greater competitiveness and promiscuity. Ardipithecus' bones took it up another notch, to 0.899. Two even older primate ancestors, Hispanopithecus and Pierolapithecus, had ratios of 0.848 and 0.908, which means they would have been tough to live with as well.

On the other end of the spectrum, the monogamous gibbons had a 1.009 ratio ... and the australopith sample came in with a ratio higher than that of modern-day humans (0.979). The implication, then, is that australopiths were monogamous.

The big question is whether there's anything substantial to this analysis. Nelson acknowledged that the fossil record was sparse, and that more fossils were needed for study, but she insisted that "this method could prove to be an exciting new way of understanding how our social behavior has evolved."

Other researchers have tried to make guesses about the social behavior of extinct hominid species by looking at sexual dimorphism -- that is, the differences between the male and the female of a species. If the males were significantly larger, the assumption is that they were built to have their way with many females in a promiscuous social setting. This has generated a fair amount of debate, particularly when it comes to assumptions about australopiths.

Nelson and her colleagues suggest that the finger-length ratio could serve as an additional tool for making more educated guesses about ancient social behavior.

"Social behaviors are notoriously difficult to identify in the fossil record," one of Nelson's fellow authors, the University of Oxford's Susanne Shultz, said in the news release. "Developing novel approaches, such as finger ratios, can help inform the current debate surrounding the social systems of the earliest human ancestors."

When this research first came to light last year, University of Wisconsin anthropologist John Hawks cautioned against reading too much into fossilized fingers. He said the index-to-ring ratio "may be correlated with mating system in primates, but that doesn't mean it's a good predictor of mating system. ... As fossil hominins go, I wouldn't expect the story to go any further -- there just aren't many hands, so there's never going to be a significantly predictive result."

Be sure to read Hawks' posting from last year, and feel free to weigh in with your comments below ... that is, after you've finished checking out your fingers.

"The 2D:4D ratio really is a subject of a lot of research in psychology and developmental biology, and it really does reflect prenatal hormone exposure. However, it is not a good predictor of any social behaviors.

"In addition to the problem of using a poor predictor, this study has another problem that we often face with fossils -- there are very few of them, and it's not obvious which sample of living primates we should be comparing with them.

"The living apes do vary in mating structure, but they also have huge differences in hand anatomy because of locomotion. Those anatomical differences between species do not necessarily have any relationship to the neurological correlates of prenatal hormones -- even though the variation in hormone exposure is an important cause of variation within species.

"For example, Ardipithecus has fingers and hand proportions that are right within the range of other apes. So when we see that they have a 2D:4D ratio right in the range of other apes, the natural hypothesis is that this reflects their overall hand proportions. Australopithecus, Neandertals and modern humans obviously had humanlike hand proportions, and their 2D:4D ratio may simply reflect this.

"If you were going to do this study right, you would look far beyond the apes to take in many kinds of primates with different social systems. Then you could see whether closely related species have 2D:4D ratios that track their mating systems. Without this, we are really looking at a single evolutionary event -- the rise of the hominins -- which may be unique for many reasons besides mating system."

In addition to Nelson and Shultz, the authors of "Digit Ratios Predict Polygyny in Early Apes, Ardipithecus, Neanderthals and Early Modern Humans but Not in Australopithecus" include Campbell Rolian and Lisa Cashmore.